Eutrophication of waters and climate warming have created in the last decades favourable conditions for cyanobacteria colonization. The presence of cyanobacteria toxins in heavily polluted lake waters and fish ponds has become a current problem. These toxins belong to cyanobacteria secondary metabolites and are active in various fields of harmfulness to animals and humans. This group includes neurotoxins, dermatotoxins and hepatotoxins having a destructive influence on liver's cells. The group of hepatotoxins comprises microcystins and nodularin. The symptoms of hepatotoxin poisoning include stomach, intestine and liver disorders, intra-liver bleeding and physiological insufficiency of this organ.These compounds can induce apoptosis of liver cells and tumor promoters. From the above facts, it follows that hepatotox-ins can pose a very serious health problem on a global scale. This work presents the characteristic of cyanobacterial hepato-toxins, their chemical structure, properties, and mechanism of their action on human organism. The harmful influence caused by consuming products used in diet supplements, which contain microcystins was also pointed out.